Electronic standby defibrillator

ABSTRACT

A METHOD AND MEANS FOR AUTOMATICALLY DEFIBRILLATING A MALFUNCTIONING HEART. WITH THE PRESENT INVENTION, THE HEART FUNCTION IS CONTINUOUSLY MONITORED. WHEN THE FUNCTION BECOMES ABNORMAL, THE MALFUNCTIONING HEART IS AUTOMATICALLY SHOCKED BY A VOLTAGE OF SUBSTANTIAL SIZE. IF THE HEART DOES NOT RETURN TO ITS NORMAL FUNCTIONS AFTER A GIVEN INTERVAL, THEN IT IS AGAIN SHOCKED. NORMAL HEART ACTIVITY ENSURES THAT THE SHOCKING MECHANISM REMAINS INERT.

Oct. M M|ROWSK| ET AL 3,614,954

ELECTRONIC STANDBY DEFIBRILLATOR Filed Feb. '9, 1970 2 Sheets-Shoot iTIME BSVl'IOA T|ME RIGHT VENTRICULAR PRESSURE CURVE INVENTORS MIECZYSLAWMIROWSKI MORTON M. MOWER WILLIAM S. STAEWEN ATTORNEYS M. MIROWSKI ET AL3,614,954 ELECTRONIC STANDBY DEFIBRILLATOR Oct. 26, 1971 mm ww $8586 o2. Sheets-Sheet 2 INVENTORS Filed Feb. 1970 MIECZYSLAW MIROWSKI MORTONM. MOWER WILLIAM S STAEWEN BYO M g ATTORNEYS United States Patent3,614,954 ELECTRONIC STANDBY DEFIBRILLATOR Mieczyslaw Mirowski, MortonM. Mower, and William S. Staewen, Baltimore, Md., assignors toMedtronic, Inc., Minneapolis, Minn.

Filed Feb. 9, 1970, Ser. No. 9,934 Int. Cl. A6111 1/36 US. Cl. 128419 D13 Claims ABSTRACT OF THE DISCLOSURE A method and means forautomatically defibrillating a malfunctioning heart. With the presentinvention, the heart function is continuously monitored. When thefunction becomes abnormal, the malfunctioning heart is automaticallyshocked by a voltage of substantial size. If the heart does not returnto its normal functions after a given interval, then it is againshocked. Normal heart activity ensures that the shocking mechanismremains inert.

BACKGROUND OF THE INVENTION During the past several decades, coronaryheart disease has come to occupy the first position among the causes ofdeath in the developed areas of the world. In the United States, forexample, this disease is responsible for over one-half million deathsyearly. And of this number, more than half occur suddenly outside thehospital, and therefore before the patient is able to obtain thenecessary medical assistance. Although the precise cause of sudden deathin coronary heart disease has not yet been entirely clarified, theavailable evidence permits the medical field to ascribe death in themajority of these cases to grave disturbances in cardiac electricalactivity resulting in ventricular fibrillation.

Recent experience has clearly demonstrated that ventricular fibrillationand its frequent precursor, ventricular tachycardia, are reversiblephenomena when prompt defibrillation of the heart is instituted. Undersuch circumstances, cardiac function can frequently be restored tonormal without the patient suffering from residual disability.Unfortunately, however, the state of the art makes defibrillation verymuch dependent upon a highly specialized medical environment, thuslimiting such treatment to elaborately equipped modern hospitals.

At the present, therefore, a great need exists for a defibrillator whichcould be carried by those who are prone to having one of the many lifethreatening arrhythmias generally discussed above. Thus, in somepatients having coronary heart diseases, a fatal outcome from:ventricular tachycardia or ventricular fibrillation could be avoided,even in the absence of immediate medical assistance. The first step, ofcourse, is the detection of those prone to suffering from cardiacmalfunctions leading to ventricular tachycardia or ventricularfibrillation.

While it is not possible to predict with unerring exactness whichpatient suffering from coronary heart disease will be the victim ofsudden death, several high risk groups of patients can be recognized.For example, patients who have experienced myocardia infarction, eventhough they may be surviving in good health, run a substantial risk ofdying suddenly, a risk several times greater than that associated withthe general population. Further, if patients with myocardial infarctionhave a history of' serious ventricular arrhythmias and/ or of cardiacarrest, or if evidence of persistent myocardial irritability is present,it may be logically assumed that the risk of sudden death is increasedsubstantially. Patients like those described above would greatly benefitif an automatic, standby or demand defibrillator were available.

3,614,954 Patented Oct. 26, 1971 Also, such an automatic defibrillatorwould be an asset to those hospital patients who have sufferedmyocardial infarction and who have been discharged from the wellequippedcoronary care unit. Under such circumstances, the defibrillator could beimplanted temporarily for the remainder of the expected hospital stay;or the defibrillator could be permanently implanted for use both in thehospital and after discharge. And another recognizable class of patientsparticularly in need of an automatic defibrillator is the class composedof those who have not shown prior histories of myocardial infarction butwho show severe symptoms of coronary heart disease, such as ventriculararrhythrnias resistant to medical treatment or angina pectoris.

From. the brief discussion above, there should be little doubt that thepossible applications for an automatic defibrillator are numerous. And,as previously noted, there is at present no known device which meets theneed. It is toward filling this gap in medical instrumentation that thepresent invention is directed.

SUMMARY OF THE INVENTION The present invention relates to a standbydefibrillator, an electronic system which, after detecting one of theabove-noted life threatening arrhythmias, automatically defibrillatesthe heart of the user. The system of the present invention may beinstalled in patients particularly prone to develop ventriculartachycardia and/or ventricular fibrillation, either on a temporary or apermanent basis. And, because of its small size, the device of thepresent invention may be entirely implanted under the skin of thepatient, or alternatively, may be carried externally, save for thesensing electrode and one shock-applying electrode.

More particularly, the present invention relates to a device forreliably sensing the differences between a properly functioning heartand one which has suddenly developed ventricular fibrillation, and whichthen delivers a defibrillating shock to the heart in fibrillation. Thedevice is adapted to continue delivering intermittent shocks to theheart in the event that the heart fails to return to its normal behaviorpattern, and has the ability of automatically regaining sensing controlover a functional heart thereby ensuring that further shocks areinhibited after successful defibrillation has taken place.

The standby defibrillator of the present invention has as its basicelement, a capacitor capable of storing electrical energy in an amountsufficient to depolarize the human heart (on the order of 50 joules).Upon discharge of this capacitor, a shock is delivered to the heartthrough two stimulating electrodes. One of these electrodes ispositioned within the right ventricle, thereby forming the distal tip ofan intracardia catheter. This electrode is introduced through aperipheral vein. The second stimulating electrode is positioned eitheron the surface of the chest, or is sutured under the skin of theanterior chest wall or directly to the ventricular myocardium.

The capacitor is associated with a sensing circuit connected to theproximal end of the intracardiac catheter and is adapted to respond to asignal recorded at the distal end of the catheter. The signal sensed bythe catheter must, of course, be inherently related to some distinctivecharacteristic of ventricular tachycardia or ventricular fibrillation;and in a specific embodiment of the present invention, the pressure inthe right ventricle is sensed. When this pressure falls below a givenvalue, on the order of 10 to 15 mm. (Hg, the heart is malfunctioningand, therefore, the capacitor is discharged into the heart.

Between the sensing circuit and the capacitor, means are provided fordelaying the repetition of depolarizing discharges for a preset periodof time (on the order of 20 to 30 seconds). This delay is essential togive the heart the opportunity to convert spontaneously to a normalcardiac rhythm, and also to ensure that the abnormal heart conditionsare, in fact, critical. Only in the absence of a successful conversionis a subsequent shock delivered to the heart. In a particular embodimentof the present invention, the time delay is brought about with the airof a sawtooth generator, a relay and the charge time of the storagecapacitor.

Accordingly, it is the main object of the present invention to provide acompact and automatic standby defibrillator which lies dormant duringnormal heart activity but which applies a shock to the heart when theheart functions become abnormal.

It is another object of the present invention to provide a standbydefibrillator which reliably senses the difference between a normallyfunctioning heart and one that has suddenly developed abnormal function,and which then automatically delivers a defibrillating shock to theheart.

-It is a further object of the present invention to provide a standbydefibrillator which is capable of delivering multiple shocks in theevent that the heart is not successfully cardioverted by the initialshock.

It is yet a further object of the invention to provide a standbydefibrillator which automatically regains sensing control over afunctioning heart, thereby inhibiting further shocks after successfuldefibrillation.

It is still another object of the invention to provide a deviceemploying a heart-implanted catheter which may sense both fordefibrillation and for pacing the heart if required.

It is still another object of the present invention to provide a standbydefibrillator which is extremely compact and which is therefor totallyimplantable in the body of a patient.

These and other objects of the invention, as well as many of theattendant advantages thereof, will become more readily apparent whenreference is made to the following description taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view of thecombination sensing probe and shock-applying probe forming a part of thepresent invention;

FIG. 2 illustrates a typical pressure curve for the right ventricle of anormally functioning heart;

FIG. 3 is a circuit schematic of the electronics COlTlprising thestandby defibrillator of the present invention; and

FIG. 4 is a graph of voltage versus time illustrating the operation ofthe sawtooth generator forming a part of the present invention.

DETAILED DESCRIPTION OF THE INVENTION With reference first to FIG. 1,the sensing and shock delivering probes will be described. The sensingprobe is shown generally at and comprises a main body portion 12 and apressure sensitive bulb 14. Electrical connections to the bulb 14 aremade at junction box 16. One of the shock delivering probes is showngenerally at 18 and comprises a main body portion 20, a first ringelectrode 22 and a second ring electrode :24. As will be explainedbelow, the electrodes 22 and 24 are short-circuited together during theoperation of the device, forming a composite electrode shown at 26. Themain body 12 of the sensing probe 110 is in the shape of a flat ribbon,and the body of the bulb 14 is spherical. The shock delivering probe 18is substantially cylindrical.

As noted previously, the combination sensing probe 10 and shockdelivering probe 18 is, during operation, positioned in the rightventricle of the heart. These probes are introduced into the heartthrough a peripheral vein by means of surgery very similar to thatinvolved in the implantation of a pacer probe.

The shock delivering electrodes are two in number. The first electrodeis the composite electrode 26 and is carried by the shock deliveringprobe 18. The second electrode is shown at 28 and, in the preferredembodiment of this invention, is a fiat plate either placed on thesurface of the chest, sutured under the skin of the anterior chest wallor applied directly to the ventricular myocardium.

When the sensing probe 10 and the shock delivering probe 18 are insertedinto the heart, the electrodes 22 and 24 are independent of one another.At this time, conventional pacemaking signals are applied between theelectrodes 22 and 24. Since the heart responds favorably to thepacemaking signals only if the probe 18 is properly positioned, thismethod is suitable for checking the position of the probes 10 and 18.The probe location may, of course, be recognized by other methods suchas, for example, fiuoroscopy or pressure recordings. Once it isdetermined that the probes 10 and 18 are properly located, they aresecured in place and the pacemaking electronics are disconnected. Then,the electrodes 22 and 24 are externally short-circuited together, andthe electronic circuit associated with the standby defibrillator of thepresent invention is then connected to the probes 10 and 18 and theelectrode 2.8. If a pacemaking function is also to be carried out, thepacer electronics will remain connected and the step of shortingtogether the electrodes 22 and '24 will be eliminated.

With reference now to FIG. 2, there is illustrated a right ventricularpressure curve for a normally functioning heart. Pulses 30 and 32 areillustrated but, as is well known, these pulses repeat at the rate ofapproximately to per minute in a normally functioning heart. FIG. 2clearly shows that each pulse has a peak and that these peaks rise abovea preset pressure indicated by the dotted line 38. This dotted linecorresponds to the threshold between a healthy heart and one which is inneed of defibrillation. When the height of the peaks 34 and 36 fallbelow the pressure indicated by line 38, the malfunction is sensed byprobe 10 which, as will be described immediately below, initiates thedefibrillation of the heart.

With reference then to FIG. 3, the electronics associated with thestandby defibrillator will be described. The electronic circuitry ofFIG. 3 may conveniently be broken down into several component parts. Thefirst part is a pressure transducer shown at 40, this pressuretransducer being directly associated with the pressure sensing probe 10shown in FIG. 1. The next state of the electronics is an amplifier shownat 42 and adapted to amplify the signals received from the pressuretransducer 40. The amplified signal from the amplifier 42 is then passedto a sawtooth generator shown at 44, which generator, in turn feeds itsoutput signal to the base of a transistor associated with the relaystage shown at 46. The relay 46 is normally in its open state conditionbut, when it is closed, a DC signal is impressed upon a DC/DC converterstage 48. The DC/DC converter 48 boosts the input voltage fromapproximately 15 volts to approximately 2,500 volts. The 2,5 00 volt DCsignal from the converter 48 is then fed to a storage capacitor 70 whichis associated with a firing circuit, the entire combination shown at 50.When the firing circuit 50 allows the capacitor 70 to discharge, the2,500 volt signal is applied to the electrodes 26 and 28 illustrated inFIG. 1. Therefore, when the pressure sensing probe 10 recognizes amalfunction in the heart, the capacitor, after a predetermined timedelay, shocks the heart with approximately 2,500 volts. This voltagecorresponds to approximately 50 joules of power, enough power to causemost hearts to defibrillate.

Still referring to FIG. 3, but in greater detail, the circuitryassociated with the present invention functions as follows. The pressuretransducer 40 takes the form of a resistive bridge, one resistor ofwhich is defined by the pressure sensor 14 on the tip of the probe 10.The remaining legs in the bridge are defined by resistor housed in thejunction box 16 shown in FIG. 1. The pressure transducer 40 is arrangedso that the pressure sensed by element 14 is converted to an electricalsignal, the amplitude of which is directly proportional to the pressuresensed by the element 14.

The output from the pressure transducer '40 is fed to a conventionalamplifier 4 2 which amplifies the received pulses and which then feedsthese amplified pulses to the sawtooth generator 44. The trimmingpotentiometer 52 seems to balance the inputs to the associatedamplifier.

With reference now to FIGS. 2 through 4, the operation of the sawtoothgenerator 44 will be described. The sawtooth generator 44, if unaffectedby the external environment, will have an output curve such as thatshown at 54 in FIG. 4. However, if a signal is fed to the sawtoothgenerator, via lead 56, and if the signal is at least of a predeterminedamplitude, then the output voltage of the generator will immediatelydrop to zero and then again begin to climb. Therefore, if the sawtoothgenerator receives repititious pulses of at least the predeterminedvoltage, then its output will be similar to that of curve 58 shown inFIG. 4.

If the heart functions sensed by the pressure transducer 40 are normal,following the curve shown in FIG. 2, then the amplified signalcorresponding to a pulse in the right ventricular pressure will causethe output of the sawtooth generator 44 to drop to zero. The thresholdsignal reaching the generator via lead 56, can be adjusted by adjustingthe amplification factor of the signal amplifier 42. This threshold isadjusted so that the generator 44 activates the relay 64 only afterapproximately six seconds of heart malfunction. If, then, theventricular pressure falls lower than that value indicated by the dottedline 38, and so remains for the preset time interval, the amplifiedvoltage reaching the generator 44, via lead 56, will be insufficient tocause the generator output to drop to zero. Rather, the generator outputwill follow the curve shown at 54 in FIG. 4. Trimming potentiometer 60is provided to balance the inputs to the associated amplifier.

The output from the sawtooth generator 44 1s fed to the relay circuit46. The relay contacts shown generally at 64 are initially set in theopen-circuit condition, thereby isolating the 15 volt source from theDC/DC converter 48. Further, the relay 46 is set to close only after thecurrent passing through coil 66 reaches a predetermined value. Withreference to FIG. 4, the voltage output of the sawtooth generator 44must be at the level 68 before the current in the coil 66 is sufficientto switch the relay 64 into its closed-circuit state.

When the relay 64 closes, then the 15 volt source is connected directlyto the DC/DC converter 48. From FIGS. 2 through 4, it should be evidentthat approximately six seconds must elapse, with the heart continu ouslymalfunctioning, before the relay 64 switches from its open-circ'uit modeto its closed-circuit mode. This will be apparent when one realizes thateach tooth of the curve 58 corresponds to one peak of the rightventricular pressure curve and, as noted above, the peaks of thepressure curve repeat at approximately 60' to 7 per minute. Therefore,the heart pressure must be below the threshold level for approximatelysix seconds before input voltage is fed to the DC/DC converter 48. Ifthe heart returns to its normal function at any time during that sixseconds, then the sawtooth generator output response would drop to zeroand the six second cycle would begin again.

With the relay 64 closed and a 15 volt DC signal being impressed uponthe converter 48, an output of 2,500 volts appears at the outputterminals of the converter 48. This voltage is fed directly to storagecapacitor 70. Simultaneously, the 2,500 volt signal is fed to aresistive chain and finally to the base of transistor 72 via a neon 6tube 74. A silicon controlled rectifier (SCR) is triggered on whentransistor 72 becomes conductive.

The operation of the firing circuit 50 is as follows: the 2,500 voltsignal from the converter 48 is fed to the capacitor 70. When thecapacitor 70 is fully charged, the transistor 72 becomes conductive, dueto the nowcondu'cting neon tube 74. The resistor chains and the tube 74are interconnected in such a manner that when the voltage across thecapacitor 70 reaches the full 2,500 volts, then the tube 74 becomesconductive. When the tube 74 conducts, so too does transistor 72 and,therefore, SCR 76. Then, the full 2,500 volts pass through electrodes 26and 28 thus shocking the heart with a voltage suflicient to causedefibrillation.

As above noted, it is important that a time period elapse between thedetection of a heart malfunction and the delivery of the defibrillatingshock to the heart. As also noted above, approximately six seconds ofdelay occur between the first detection of a malfunction and the closingof the relay 64. There is an additional delay, on the order of fifteenseconds, which is brought about by the charge time of the capacitor 70.That is, when the relay 64 closes, six seconds after the initialmalfunction, the capacitor first begins to charge. The capacitoremployed in the preferred embodiment charges in approximately fifteenseconds. Therefore, approximately twenty-one seconds elapse between theinitial sensing of heart malfunction and the discharge of the capacitorinto the heart. Naturally by varying the rise time of the sawtoothgenerator and the charge time of the capacitor, the twenty-one secondsmay be enlarged or contracted as desired. And, as mentioned above, if atany time during the delay period the heart returns to normal, then thedelay period automatically begins again.

Above, a specific embodiment of the present invention has beendescribed. It should be understood, however, that this description isgiven for illustrative purposes only and that many alterations andmodifications may be practiced Without departing from the spirit andscope of the invention. Just as a few examples, it should be understoodthat while in the specific embodiment of the present invention, thepressure in the right ventricle is sensed as an indication of heartmalfunction, other sensing arrangements may be practiced. Further, asingle SCR is used as a triggering device. It is possible to substitutethis device for a plurality of SCR units or, alternatively, with avacuum relay. Still further, while the above description shows a sing-1estorage capacitor, a series of capacitors could be employed. It is,therefore, the intent that the present invention not be limited to theabove but be limited only as defined in the appended claims.

We claim:

1. An electronic device for automatically defibrillating amalfunctioning heart, the device comprising: electronic probe means forcontinually sensing the function of a heart and for issuing anelectrical signal, the amplitude of which is proportional to the heartfunction; discriminator means associated with said probe means forresponding to the electrical signal issued thereby and for issuing anelectrical signal of a first amplitude when the heart function is normaland for issuing an electrical signal of a second amplitude when theheart function is abnormal; means for storing electrical energyelectrode means associated with said storage means for connecting thestorage means directly to the heart; and means for automaticallyswitching said storage means into a discharge state, in response to thesignal of said second amplitude, whereby the stored energy is applieddirectly to the heart through said electrode means to defibrillate amalfunctioning heart.

2. The device as set forth in claim 1, and further comprising: delaymeans for ensuring that a time delay exists between the sensing of theinitial heart malfunction and the discharge of said storage means intothe heart.

3. The device as set forth in claim 1, wherein said sensing means isadapted to be positioned within the heart.

4. The device as recited in claim 1, wherein the means for applying thestored power to the heart comprises two electrodes, one of which isadapted to be positioned within the heart.

5. The device as set forth in claim 2 wherein said switching meansincludes: means for inhibiting the discharge of said storage means inresponse to a signal of said first amplitude occurring during said timedelay.

6. The device recited in claim 1, wherein said electronic probe includespressure responsive means which senses the right ventricular pressureand issues an electrical signal whose amplitude is directly proportionalto the pressure.

7. The device of claim 1, wherein said discriminator means includessawtooth generator means having small amplitude output means responsiveto normal heart function and large amplitude output means responsive toabnormal heart function, and time delay means connected to the largeamplitude output means for delaying large amplitude output for a presetperiod of time.

8. The device recited in claim 7, wherein said switching means comprisesrelay means connected to said sawtooth generator means and havingnormally open contact means adapted to close only when subjected to saidlarge amplitude signal from said discriminator means.

9. The device of claim 8, wherein said means for storing comprises:voltage generator means for generating a voltage sufficient todefibrillate the heart, said voltage generator means operativelyconnected to said contact means for operating only when said contactmeans are closed.

10. The device of claim 9 including a capacitor connected directly tothe output of said voltage generator means.

1 1. The device of claim 10, and further comprising: switching meanshaving a normally open-circuit state, said switching means connected tosaid capacitor for switching to a closed-circuit state after saidcapacitor has fully charged, the closed-circuit state of said switchingmeans resulting in the discharge of said capacitor into the heart. I

12. The device of claim 11, wherein said switching means includes gaslamp means responsive to the charge on said capacitor to operate saidswitch means.

13. The device of claim 1, wherein said means for automaticallyswitching includes at least one silicon C011? trolled rectifier.

References Cited UNITED STATES PATENTS 3,138,151 6/1964 Chapman et al.128-205 3,236,239 2/1966 Berkovits 128-419 3,358,690 12/1967 Cohen128419 3,481,341 12/1969 Siedband 128421 OTHER REFERENCES Germanapplication, Frank, 1,067,538, Oct. 22, 1959.

WILLIAM E. KAMM, Primary Examiner US. Cl. X.R. 1282.06 F, 404

